In packet-switched transmission the information to be transmitted over a network is split into small data units called packets. The packets, which include the recipient's address information, are transferred from the sender to the recipient by routing their route in the network according to the recipient's address. In packet-switched transmission the same radio resources can be divided between several users according to the need.
The GPRS is a packet-switched telecommunications service of the GSM network (Global System for Mobile Communications) which supplements the existing services, such as conventional circuit-switched communication and short message service (SMS). In conventional circuit-switched communication between a wireless terminal, such as a mobile station or a computer terminal, and a base station subsystem (BSS) radio resources are typically reserved by reserving a physical (radio) channel for the duration of a call, the physical channel referring to a certain time slot of the transmission frame on a certain frequency band. The GPRS enables dynamic reservation of physical channels for transmission. In other words, a physical channel is reserved for a certain MS-BSS link only when data are being transmitted. Thus unnecessary reservation of radio resources can be avoided when there are no data to be transmitted.
The GPRS is intended to function together with circuit-switched transmission of a conventional GSM network to use the air interface efficiently for data and speech communications. For this reason, the GPRS employs the basic channel structure defined for the GSM. In the GSM a certain frequency band is divided into a string of transmission frames in the time domain which are known as TDMA frames (Time Division Multiple Access). The duration of a TDMA frame is 4.615 ms. Each TDMA frame is divided into eight successive time slots which are equal in duration. When a call is set up in the conventional circuit-switched transmission mode, a physical channel is defined for this call by reserving a certain time slot (1 to 8) in each TDMA frame string. Physical channels are defined in a corresponding manner for carrying different signallings over the network.
Radio resources are reserved for transmission by assigning the physical channels dynamically either to a circuit-switched or a packet-switched transmission mode. When the circuit-switched transmission mode sets high requirements on the network, a large number of time slots can be reserved for this transmission mode. On the other hand, when the demand for the GPRS transmission mode is great, a large number of time slots can be reserved for this transmission mode.
A terminal which can at the same time have only a circuit-switched or a packet-switched radio connection to a base station of the base station subsystem BSS is called a class B GPRS terminal. This means that if the terminal is in the circuit-switched operation mode (e.g. the terminal is involved in a circuit-switched call), it can neither transmit nor receive packet-switched data, and vice versa. This poses a problem which will be illustrated in the following with reference to FIG. 1, which illustrates some elements of a telecommunications network that are needed to implement circuit-switched and packet-switched services.
The main element of the network infrastructure for GPRS services is a GPRS support node, which in packet-switched transmission corresponds to a mobile switching centre MSC of the GSM network known from circuit-switched transmission. The GPRS support nodes of the GPRS network are divided into serving GPRS support nodes SGSN and gateway GPRS support nodes GGSN. The SGSN is a support node which sends data packets to a terminal MS and receives data packets sent by the terminal MS via a base station subsystem BSS consisting of base stations BTS and base station controllers BSC. The SGSN also maintains, together with GPRS registers (not shown in the figure), location data on terminals that roam in its service area in the GPRS network. The SGSN is typically implemented as a physically separate network element. A GGSN communicating with an SGSN implements switching and cooperation with IP networks 13 (Internet Protocol). Such IP networks include Internet and/or Intranet networks. Reference numeral 12 denotes a server of the IP network which can transmit and receive packet-switched data via the GPRS network.
The mobile switching centre MSC provides circuit-switched services for the terminal MS. One example of circuit-switched services is a circuit-switched call to a telephone 11 of a public switched telephone network PSTN. The call passes between the terminal MS and the telephone 11 via the base station subsystem BSS and the MSC. It is also possible to access IP networks 13 from the PSTN. The radio interface between the terminal MS and the base station subsystem BSS is called a Um interface.
It is now assumed that the terminal MS is in the packet-switched GPRS operation mode and has a TCP/IP connection (Transmission Control Protocol) to a server 12 of the IP network. The server 12 sends packets (TCP/IP packets) from the IP network 13 to the terminal via the GGSN, SGSN and BSS according to the TCP and IP protocols. The amount of data to be transmitted is controlled with the transmission window size. The server 12 may transmit one or more TCP/IP packets at the same time. The terminal MS acknowledges the packets it has received from the server 12 by sending TCP acknowledgement messages to the server according to the TCP protocol, the TCP acknowledgement messages indicating to the server 12 which TCP/IP packets the terminal MS has received.
Even though the terminal MS cannot transmit and receive circuit-switched data when in the packet-switched operation mode, it can, however, receive circuit-switched paging messages. Paging messages are signalling messages which are transmitted to the terminal to signal that there is an incoming call for the terminal.
It is now assumed that a call is made from the telephone 11 of the public switched telephone network PSTN to the terminal MS when it has an ongoing TCP/IP connection with the server 12. In that case the MSC sends a paging message to the terminal. When the terminal receives the paging message, it switches to the GPRS suspend state to allow the terminal user to answer the circuit-switched call. The terminal switches from the GPRS suspend state (circuit-switched operation mode) back to the packet-switched operation mode when the circuit-switched call ends. The duration of a circuit-switched call may be from a few seconds to dozens of minutes. During the call the terminal is in the GPRS suspend state. In that case the TCP/IP connection suffers badly because the terminal cannot transmit or receive packet-switched data in the suspend state, i.e. it cannot receive the TCP/IP packets sent by the server 12 nor send TCP acknowledgement messages to the server.
In general, when the sender transmits TCP/IP packets to the recipient on a TCP/IP connection, a retransmission timer is set according to the TCP protocol. If acknowledgement messages sent from the receiving end do not reach the sender within a set time limit, the retransmission timer expires. The TCP protocol takes care that unacknowledged TCP/IP packets are retransmitted to the recipient. In practice, retransmission is implemented as follows: if the sender does not receive an acknowledgement/acknowledgements to a packet/packets sent, it:                doubles the value of the retransmission timer,        sets 1 MSS (Maximum Segment Size) as the size of its TCP transmission window, and        retransmits the first unacknowledged TCP/IP packet to the receiving end.        
The size of the transmission window is limited to 1 MSS in retransmission to ensure that a large amount of data which might not be received is not sent to the transmission path.
When the terminal MS switches to the GPRS suspend state in the case of an incoming circuit-switched call, the retransmission timer of the server 12 expires because the terminal cannot transmit or receive packet-switched data in the suspend state, and thus the server 12 does not receive acknowledgement messages from the terminal MS within the set time limit. Now the server 12 doubles the value of its retransmission timer, which may originally have been a few seconds, and retransmits the first unacknowledged TCP/IP packet (transmission window size=1 MSS). If the circuit-switched call is still going on, the server does not receive an acknowledgement to this TCP/IP packet, either, and thus the retransmission timer expires again. The server 12 doubles the value of its retransmission timer again and retransmits the TCP/IP packet, etc.
The retransmission timer may expire several times, finally reaching its maximum value, which is usually 60 seconds. If the circuit-switched call is still going on, retransmissions are continued until their number exceeds a preset maximum, in which case the TCP/IP connection is finally aborted.
When the circuit-switched call ends, the terminal returns to the packet-switched operation mode (e.g. by means of a resume procedure), and thus it can again transmit and receive packets of the TCP/IP connection. However, the TCP/IP connection may have been aborted in between, and the data that were transmitted earlier during the TCP/IP connection in question may be lost. Alternatively, even though the TCP/IP connection has not been aborted, unnecessary additional delay is caused before normal transmission of TCP/IP packets can continue. For example, if the retransmission timer has reached its maximum value, it may take even about 60 seconds until any packets are transmitted. This is not optimal use of network resources.